This application relates to and incorporates herein by reference Japanese Patent Application No. 11-205759 filed on Jul. 21, 1999.
1. Field of the Invention
The present invention relates to an air-fuel ratio control for engines which improves air-fuel mixture supply immediately after completion of mixture combustion for engine starting.
2. Related Art
Conventional engine control systems have a catalytic converter in an exhaust pipe to purify exhaust emissions, and feedback-controls an air-fuel ratio of air-fuel mixture to a stoichiometric ratio in response to the air-fuel ratio detected by an air-fuel ratio sensor. The feedback control is disabled until engine temperature sufficiently rises as disclosed in JP-A-60-3440, because the air-fuel ratio sensor is not operative under low temperatures. Therefore, the feedback control is disabled during an engine starting (cranking by a starter motor) period and a post-starting period.
Further, immediately after the complete combustion of air-fuel mixture in those starting and post-starting periods, the engine rotation speed quickly rises and then falls, thus presenting irregular rotation speed changes. If less-volatile heavy fuel is supplied to the engine, the fuel is likely to remain sticking to intake port walls of the engine during low temperature conditions, thus leaning the air-fuel mixture supplied to the engine. The engine may misfire and stall immediately after engine starting.
It is therefore an object of the present invention to provide an air-fuel ratio control for engines which controls an air-fuel mixture ratio appropriately immediately after a complete mixture combustion in an engine starting operation.
According to the present invention, a target operation characteristics of engine speed is set based on a coolant temperature at the time of starting an engine cranking. The target engine speed is variable with time after starting engine cranking. Immediately after a complete combustion of air-fuel mixture supplied to the engine is detected, an actual engine speed is compared with a target engine speed corresponding to the target operation characteristics, and an air-fuel ratio of mixture supplied to the engine is controlled based on a comparison result. Thus, the air-fuel ratio control is effected immediately after the complete combustion of air-fuel mixture, even when an air-fuel ratio sensor is inoperative to effect an air-fuel ratio feedback control.
Preferably, the target engine speed is determined to converge to a speed value lower than a normal target idle speed, and the air-fuel ratio control based on the comparison result prevails an engine idle speed feedback control. The air-fuel ratio control is effected by using a first correction value calculated as a function of a difference between the target speed and the actual speed, and a second correction value calculated as a function of a difference between the target speed and an estimated future speed estimated from air flow amount. The air-fuel ratio control is further effected by using a combustion unstableness value.